1. Field of the Invention
This invention relates to gelled liquid hydrocarbon fluids and methods of their use and preparation.
2. Description of the Prior Art
High viscosity gelled hydrocarbon liquids have heretofore been utilized in treating subterranean formations penetrated by well bores, e.g., hydraulic fracturing stimulation treatments. In such treatments, a high viscosity gelled liquid hydrocarbon fracturing fluid having particulate proppant material, e.g., sand, suspended therein is pumped through a well bore into a subterranean formation to be stimulated at a rate and pressure such that one or more fractures are formed and extended in the formation. The suspended proppant material is deposited in the fractures when the gelled hydrocarbon fracturing fluid is broken and returned to the surface. The proppant material functions to prevent the formed fractures from closing whereby conductive channels remain through which produced fluids can readily flow to the well bore.
Polyvalent metal salts of orthophosphoric acid esters have heretofore been utilized as gelling agents for forming high viscosity gelled liquid hydrocarbon fracturing fluids. Such gelled liquid hydrocarbon fracturing fluids have included fracture proppant material and delayed breakers for causing the fracturing fluids to break into relatively thin fluids whereby the proppant material is deposited in formed fractures and the fracturing fluid is produced back. Descriptions of such heretofore utilized high viscosity gelled liquid hydrocarbon fracturing fluids and methods of their use are set forth in U.S. Pat. No. 4,622,155 issued to Harris et al. on Nov. 11, 1986, and U.S. Pat. No. 5,846,915 issued to Smith et al. on Dec. 8, 1998. The gelled liquid hydrocarbon fracturing fluids described in the above patents utilize ferric iron or aluminum polyvalent metal salts of phosphoric acid esters as gelling agents and delayed breakers such as hard burned magnesium oxide which is slowly soluble in water.
While the heretofore utilized high viscosity gelled liquid hydrocarbon fracturing fluids and methods have been used successfully for forming fractures in subterranean formations, problems have been encountered as a result of the use of the gelling agent, i.e., the polyvalent metal salt of a phosphoric acid ester. That is, in recent years plugging of refinery towers which process oil produced from formations fractured with gelled liquid hydrocarbon fracturing fluids has caused many expensive, unplanned shut-downs. The plugging material is high in phosphorus and has been attributed to the phosphate esters used as gelling agents. The phosphate esters contribute volatile phosphorus which condenses on distillation tower trays, causing plugging. The volatile phosphorus may also carry over the tops of the distillation towers causing contamination of the hydrocarbon products produced.
Thus, there are needs for improved methods of using and preparing gelled liquid hydrocarbons which upon breaking and being refined substantially reduce volatile phosphorus in distillation towers, improved liquid hydrocarbon gelling agents and improved gelled liquid hydrocarbon compositions. More specifically, in fracturing oil producing subterranean formations in areas where volatile phosphorus is a problem in refineries, there is a need to reduce the production of volatile phosphorus in the refineries to levels where the above described unscheduled refinery shut downs are not required without compromising CO2 compatibility with the gelled oil fracturing fluids used. Concentrations of CO2 as high as 40-50% are commonly used in gelled oil fracturing fluids which form miscible mixtures with the fluids. The presence of the CO2 enhances fluid recovery, minimizes the amount of oil based fracturing fluid which must be recovered, and reduces costs in areas where CO2 is less expensive than the oil based fracturing fluid.
The present invention provides improved methods of using and preparing gelled liquid hydrocarbons, improved liquid hydrocarbon gelling agents and improved gelled liquid hydrocarbon compositions which meet the above described needs and overcome the deficiencies of the prior art.
The improved methods of this invention for fracturing subterranean formations using gelled liquid hydrocarbons are basically comprised of the following steps. An improved gelled liquid hydrocarbon fracturing fluid is prepared comprised of a hydrocarbon liquid, a gelling agent comprised of a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester, a proppant material, water and an amount of a delayed gel breaker effective to break the gelled hydrocarbon fracturing fluid. The phosphonic acid ester utilized in the gelling agent which minimizes volatile phosphorus in refinery distillation towers has the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms. After the gelled liquid hydrocarbon fracturing fluid is prepared, the subterranean formation to be fractured is contacted with the gelled liquid hydrocarbon fracturing fluid under conditions effective to create at least one fracture in the subterranean formation.
The improved methods of this invention for preparing gelled liquid hydrocarbons basically comprise adding a phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms, and preferably, at least a stoichiometric amount of a polyvalent metal source selected from ferric iron salts and aluminum compounds to a hydrocarbon liquid. The polyvalent metal source reacts with the phosphonic acid ester to form a ferric iron or aluminum polyvalent metal salt thereof. Water and an amount of a delayed gel breaker effective to break the gelled liquid hydrocarbon fracturing fluid is also added to the hydrocarbon liquid.
The improved liquid hydrocarbon gelling agents of this invention are comprised of a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms.
The improved gelled liquid hydrocarbon compositions of this invention are comprised of a hydrocarbon liquid, a gelling agent comprising a polyvalent metal salt of a phosphonic acid ester produced from a phosphonic acid ester and a ferric iron salt or an aluminum compound, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms, water and a delayed gel breaker present in an amount effective to break the gel formed by the gelling agent and the hydrocarbon liquid.
It is, therefore, an object of the present invention to provide improved methods and compositions useful in treating subterranean formations as well as in other similar applications.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
The present invention provides methods of treating subterranean formations penetrated by well bores with gelled liquid hydrocarbon fluids. For example, the gelled liquid hydrocarbon fluids are suitable for use in fracturing treatments carried out in subterranean formations whereby the production of hydrocarbons from the subterranean formations is stimulated. In accordance with the present invention, a fracturing fluid comprised of a gelled liquid hydrocarbon or mixture of hydrocarbons containing a proppant material and a delayed gel breaker is pumped through a well bore into a subterranean formation to be stimulated. The fracturing fluid is pumped at a rate and pressure such that one or more fractures are formed and extended in the subterranean formation. The proppant material which is suspended in the fracturing fluid is deposited in the fractures when the gel is broken and returned to the surface. The proppant material remains in the fractures and functions to prevent the fractures from closing whereby conductive channels are formed through which produced fluids can readily flow from the subterranean formation into the well bore.
As mentioned above, gelled liquid hydrocarbon fracturing fluids have heretofore been formed with a gelling agent comprised of a ferric iron or aluminum polyvalent metal salt of a phosphoric acid ester. The phosphoric acid ester suffers from the problem that it decomposes in refinery distillation towers to form volatile phosphorus which condenses on the trays of the distillation towers and causes plugging. Also, the phosphoric acid ester may itself be volatile, dependent upon its molecular weight. By the present invention, improved methods and compositions for fracturing subterranean formations with gelled liquid hydrocarbon fracturing fluids are provided wherein the gelling agent utilized is a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester. Unlike the phosphoric acid esters utilized heretofore, the phosphonic acid esters of the present invention have much higher thermal stability and consequently do not as readily decompose or disassociate in refining towers.
Thus, the improved methods of fracturing subterranean formations of the present invention are basically comprised of the following steps. A gelled liquid hydrocarbon fracturing fluid is prepared comprised of a hydrocarbon liquid, a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester, a proppant material, water and an amount of a delayed gel breaker effective to break the gelled liquid hydrocarbon fracturing fluid. The phosphonic acid ester has the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms. After the gelled liquid hydrocarbon fracturing fluid is prepared, the subterranean formation to be fractured is contacted with the fracturing fluid under conditions effective to create at least one fracture in the subterranean formation.
The hydrocarbon liquid utilized to form the gelled liquid hydrocarbon fracturing fluid can be any of the various previously used hydrocarbon liquids including, but not limited to, olefins, kerosene, diesel oil, gas oil (also known as gas condensate), fuel oil, other petroleum distillates, and certain mixtures of crude oil. Liquid hydrocarbon fracturing fluids which are specifically designed for use with CO2 are generally preferred. Such a liquid hydrocarbon fracturing fluid is commercially available from the Trysol Corporation of Sundre, Alberta, Canada under the trade name xe2x80x9cFRACSOL(trademark).xe2x80x9d
As mentioned above, the gelling agent utilized for gelling the hydrocarbon liquid whereby it has a high viscosity sufficient to carry suspended proppant material and produce fractures in a subterranean formation is a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester having the formula set forth above. The polyvalent metal salt of the phosphonic acid ester is preferably produced at the well site by adding the phosphonic acid ester, and preferably, at least a stoichiometric amount of a polyvalent metal salt (preferably a ferric iron salt or an aluminum compound). In addition, if water is not already contained in the hydrocarbon liquid or added thereto as a component in a cross-linker solution or the like, water is added to the hydrocarbon liquid in an amount, for example, of about 0.05% or greater by weight of the hydrocarbon liquid. The presence of the water allows slowly water soluble or encapsulated breakers to be dissolved or released. See, for example, Smith et al. U.S. Pat. No. 5,846,915 issued on Dec. 8, 1995 which is incorporated herein by reference.
When a ferric iron salt is utilized to form the gelling agent, it is preferably selected from ferric sulfate or ferric chloride with ferric sulfate being preferred. The ferric iron salt is typically mixed with amines, surfactants and water to form a liquid cross-linking solution. An example of a commercially available ferric iron cross-linking solution is xe2x80x9cEA-3(trademark)xe2x80x9d cross-linking solution sold by Ethox Chemicals, Inc. of Greenville, S.C. When an aluminum compound is utilized, it is preferably selected from aluminum chloride or aluminum isopropoxide, with aluminum chloride being the most preferred. The polyvalent metal compound utilized reacts with the phosphonic acid ester to form the hydrocarbon liquid gelling agent of this invention which gels the hydrocarbon liquid. The phosphonic acid ester is added to the hydrocarbon liquid along with the polyvalent metal source to form the gelling agent in the hydrocarbon liquid in an amount in the range of from about 0.1% to about 2.5% by weight of the hydrocarbon liquid.
As mentioned above, the phosphonic acid ester which can be utilized to form the hydrocarbon liquid gelling agent of this invention has the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms. Techniques which can be utilized for the preparation of the phosphonic acid esters useful in accordance with this invention are, for example, described in U.S. Pat. No. 3,798,162 issued to Dickert, Jr. on Mar. 19, 1974 which is incorporated herein by reference.
Proppant materials which can be utilized in the fracturing fluids of this invention are well known in the art. For example, proppant materials such as graded sand, resin coated sand, sintered bauxite, various particulate ceramic materials, glass beads and the like can be utilized. The particular size of the proppant material employed depends on the particular formation being fractured and other variables. Generally, the proppant particle sizes are in the range of from about 2 to about 200 mesh on the U.S. Sieve Series scale.
A variety of delayed gel breakers can be utilized in accordance with the present invention to cause the gelled liquid hydrocarbon fracturing fluid to revert to a thin fluid that is produced back after fractures are formed in a subterranean formation. The gel breakers can be materials which are slowly soluble in water which, as mentioned above, is combined with or otherwise present in the hydrocarbon liquid. The breaking of the gel does not take place until the slowly soluble breakers are dissolved in the water. Examples of such slowly soluble breakers are given in U.S. Pat. No. 5,846,915 issued to Smith et al. on Dec. 8, 1998 which is incorporated herein by reference. As indicated in U.S. Pat. No. 5,846,915, hard burned magnesium oxide having a particle size which will pass through a 200 mesh Tyler screen is preferred. The hard burned magnesium oxide and other similar breakers are not immediately present for breaking the gel due to their slowly soluble nature. Other breakers such as alkali metal carbonates, alkali metal bicarbonates, alkali metal acetates, other alkaline earth metal oxides, alkali metal hydroxides, amines, weak acids and the like can be encapsulated with slowly water soluble or other similar encapsulating materials. Such materials are well known to those skilled in the art and function to delay the breaking of the gelled hydrocarbon liquid for a required period of time. Examples of water soluble and other similar encapsulating materials which can be utilized include, but are not limited to, porous solid materials such as precipitated silica, elastomers, polyvinylidene chloride (PVDC), nylon, waxes, polyurethanes, cross-linked partially hydrolyzed acrylics and the like. Of the slowly soluble or encapsulated breakers mentioned, hard burned magnesium oxide which is commercially available from Clearwater Inc. of Pittsburgh, Penn. is preferred for use in accordance with the present invention. When an alkaline breaker is utilized, e.g., magnesium oxide, the acid group of the phosphonic acid ester in the gelling agent is neutralized which initially increases the viscosity of the gelled hydrocarbon liquid after which the gel is broken.
Another type of breaker which can be utilized when the gelling agent is a ferric iron polyvalent metal salt of a phosphonic acid ester of this invention, or a ferric iron polyvalent metal salt of the heretofore used phosphoric acid ester, is a reducing agent that reduces ferric iron to ferrous iron. Since only ferric iron is capable of forming a viscous coordination complex with a phosphonic acid ester or a phosphoric acid ester, the complex can be disassociated by reducing the ferric iron to the ferrous state. The disassociation causes the gelled hydrocarbon liquid to break. Examples of reducing agents which can be utilized include, but are not limited to, stannous chloride, thioglycolic acid (2-mercaptoacetic acid), hydrazine sulfate, sodium diethyldithiocarbamate, sodium dimethyldithiocarbamate, sodium hypophosphite, potassium iodide, hydroxylamine hydrochloride, thioglycol (2-mercaptoethanol), ascorbic acid, sodium thiosulfate, sodium dithionite and sodium sulfite. Of these, the preferred reducing agents for use at a temperature of about 90xc2x0 C. are stannous chloride, thioglycolic acid, hydrazine sulfate, sodium diethyldithiocarbamate and sodium dimethyldithiocarbamate. The most preferred reducing agent is thioglycolic acid which may be delayed by salt formation or encapsulation. As mentioned above in connection with other breakers that can be used, the reducing agent utilized can also be delayed by encapsulating it with a slowly water soluble or other similar encapsulating material.
The gel breaker utilized in a water-containing gelled liquid hydrocarbon fracturing fluid of this invention is generally present therein in an amount in the range of from about 0.01% to about 3% by weight of the hydrocarbon liquid, more preferably in an amount in the range of from about 0.05% to about 1%.
A preferred method of fracturing a subterranean formation in accordance with the present invention is comprised of the steps of: (a) preparing a gelled liquid hydrocarbon fracturing fluid comprised of a hydrocarbon liquid, a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms, a proppant material, water and an amount of a delayed gel breaker effective to break the gelled hydrocarbon fracturing fluid; and (b) contacting the subterranean formation with the gelled hydrocarbon fracturing fluid under conditions effective to create at least one fracture in the subterranean formation.
The ferric iron or aluminum polyvalent metal salt of the phosphonic acid ester is present in the fracturing fluid in an amount in the range of from about 0.1% to about 2.5% by weight of the hydrocarbon liquid in the fracturing fluid, more preferably in an amount in the range of from about 0.2% to about 1%. The proppant material is present in the fracturing fluid in an amount in the range of from about 1 to about 14 pounds of proppant material per gallon of hydrocarbon liquid in the fracturing fluid. As mentioned, water is added to or otherwise contained in the hydrocarbon liquid so that the delayed gel breaker utilized is dissolved in the water. The delayed gel breaker is present in the fracturing fluid in an amount in the range of from about 0.01% to about 3% by weight of the hydrocarbon liquid in the fracturing fluid, more preferably in an amount in the range of from about 0.05% to about 1%.
A preferred method of preparing a gelled liquid hydrocarbon fluid of this invention is comprised of adding a phosphonic acid ester to a hydrocarbon liquid, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms, at least a stoichiometric amount of a polyvalent metal source selected from ferric iron salts and aluminum compounds which reacts with the phosphonic acid ester to form a ferric iron or aluminum polyvalent metal salt thereof, water and an amount of a delayed gel breaker effective to break the gelled hydrocarbon fracturing fluid.
The ferric iron or aluminum polyvalent metal salt of the phosphonic acid ester formed in the hydrocarbon liquid is present therein in an amount in the range of from about 0.1% to about 2.5% by weight of the hydrocarbon liquid, more preferably in an amount in the range of from about 0.2% to about 1%. The delayed gel breaker utilized is present in the hydrocarbon liquid in an amount in the range of from about 0.01% to about 3% by weight of the hydrocarbon liquid, more preferably in an amount in the range of from about 0.05% to about 1%.
A preferred hydrocarbon liquid gelling agent of this invention is comprised of a ferric iron or aluminum polyvalent metal salt of a phosphonic acid ester, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 8 to about 24 carbon atoms and Rxe2x80x2 is an alkyl group having from about 1 to about 4 carbon atoms.
A more preferred hydrocarbon liquid gelling agent is comprised of a ferric iron polyvalent metal salt of a phosphonic acid ester, the ester having the formula 
wherein R is an alkyl group having from about 14 to about 24 carbon atoms and Rxe2x80x2 is a methyl group.
A preferred gelled liquid hydrocarbon composition of this invention is comprised of: a hydrocarbon liquid; a gelling agent comprising a polyvalent metal salt of a phosphonic acid ester produced from a phosphonic acid ester and a ferric iron salt or an aluminum compound, the phosphonic acid ester having the formula 
wherein R is an alkyl group having from about 14 to about 24 carbon atoms and Rxe2x80x2 is a methyl group, water and a delayed gel breaker present in an amount effective to break the gel formed by the gelling agent and the hydrocarbon liquid.
As mentioned above, the delayed gel breaker can be alkaline earth metal oxides including hard burned magnesium oxide, alkali metal carbonates, alkali metal bicarbonates, alkali metal acetates, alkali metal hydroxides, amines and weak acids which are slowly water soluble or are encapsulated with a slowly water soluble or other similar encapsulating material. The delayed gel breaker can also be a reducing agent that reduces ferric iron to ferrous iron encapsulated with a slowly water soluble or other similar encapsulating material. The delayed gel breaker is present in the gelled hydrocarbon liquid in an amount in the range of from about 0.01% to about 3% by weight of the hydrocarbon liquid, more preferably in an amount in the range of from about 0.05% to about 1%.
As will be understood by those skilled in the art, the use of the improved liquid hydrocarbon gelling agents and gelled liquid hydrocarbon compositions is not limited to fracturing subterranean formations. For example, the gelled liquid hydrocarbon compositions can be used for forming gravel packs in well bores, in pipeline pigging operations and in other operations where a gelled hydrocarbon liquid which subsequently breaks into a thin fluid is utilized.